A susceptor is a material that converts electromagnetic energy to thermal energy and may be used to heat various materials during, for example, a manufacturing process. A “smart” susceptor is a susceptor assembly that is self-regulating with regard to temperature. Typically, the smart susceptor is placed in an electromagnetic flux field that is generated by an inductor. Susceptor materials include various ferromagnetic materials, for example ferrous nickel-cobalt alloys such as Kovar®, as well as other alloys of iron, nickel, and cobalt.
At relatively low temperatures, the susceptor is highly permeable to the electromagnetic flux field and a cross sectional region through which electrons flow through the susceptor (i.e., the skin depth) is small. Thus, at these relatively low temperatures, an electrical resistance of the susceptor is high. When placed into the electromagnetic flux field generated, for example, by an induction coil that is part of the smart susceptor assembly, the susceptor begins to inductively heat due to the initially small skin depth and high magnetic permeability. As the susceptor heats, a thermal profile of the susceptor asymptotically approaches its leveling temperature where the susceptor maintains thermal equilibrium. The leveling temperature is typically a few degrees (e.g., within 2° F., or within 10° F., or within 50° F., or within 100° F.) below the smart susceptor's designed “Curie” temperature or “TC”, at which the susceptor becomes nonmagnetic. As the susceptor approaches its leveling temperature, the magnetic permeability of the susceptor decreases, which increases the skin depth, thereby attenuating the electrical resistance of the susceptor and reducing the heating effect. The drop in magnetic permeability limits the generation of heat at those susceptor portions at or near the leveling temperature. The magnetic permeability at a given point in time can be different for different regions of the susceptor, depending on the localized temperature at localized regions. As each localized region of the susceptor approaches the leveling temperature, the localized region becomes less magnetic until steady state (i.e., thermal equilibrium) is reached and further heating of the susceptor at the localized region ceases. Regions of the susceptor that reach the Curie temperature become nonmagnetic at or above the Curie temperature. When the susceptor begins to cool, its magnetic permeability increases, the skin depth decreases, its electrical resistance increases, and the heating process begins again.
Because of its properties of temperature self-regulation, the smart susceptor is a valuable tool in manufacturing and other uses. Some conventional designs of smart susceptors include a susceptor material wrapped around a litz wire. The litz wire can include a core with a plurality of electrically conductive strands, for example, copper strands. When an alternating current is applied to the litz wire, the litz wire generates a magnetic flux field. The susceptor absorbs the electromagnetic energy generated by the litz wire and converts it to heat. The litz wire with the susceptor wrapped therearound can be imbedded within a silicone layer to form a heat blanket that can be used, for example, to heat a carbon fiber that is pre-impregnated with an uncured resin.
Improving temperature uniformity of a heat blanket and increasing the range of leveling temperatures available for a given susceptor material would be desirable.